The inventors herein have recognized a need for an improved diagnostic system for a DC-DC voltage converter. The diagnostic system provides a technical effect of obtaining diagnostic diversity by sampling a first output voltage from a first temperature sensor using a first channel of a first bank of channels in an analog-to-digital converter, and then sampling a second output voltage from a second temperature sensor using a first channel of a second bank of channels in the analog-to-digital converter, to determine over-temperature conditions in a high voltage bi-directional MOSFET switch and a low voltage bi-directional MOSFET switch, respectively, in the DC-DC voltage converter, and if so, then inducing the high voltage bi-directional MOSFET switch and the low voltage bi-directional MOSFET switch to each transition to an open operational state.